This invention relates generally to apparatus and method for drying a coating containing a volatile liquid that has been applied to a support strip that is moved through the apparatus, and more specifically to a drying tunnel for a casting machine for casting ceramic slurry that forms self-supporting tape when dry.
The most familiar example of a coating that is applied as a liquid and dried to a solid film is paint. A housepaint is typically formulated from mineral powders that are dispersed in a liquid, called a vehicle, that has a binder such as linseed oil dissolved in a solvent such as turpentine. When the paint is applied to a surface, the solvent evaporates into the atmosphere and leaves behind the mineral powders, or pigments, that are connected into a tough film by the binder, which also helps adhere the film to the substrate surface.
The electronics industry has long used a similar principle to form thin sheets of ceramic materials to use as the insulating portion of structures and devices used in electronic circuits.
U.S. Pat. No. 2,966,719, of J. L. Park, Jr., is the earliest disclosure of forming ceramic xe2x80x9ctapexe2x80x9d on a continuous support strip. U.S. Pat. No. 6,097,135, of Cappabianca, issued in 2001, discusses the art of xe2x80x9ctape castingxe2x80x9d from a modern perspective.
The constituents of the ceramic formulation are mixed into a xe2x80x9cslurryxe2x80x9d, or xe2x80x9cslipxe2x80x9d, by dispersing them into a vehicle that contains solvent, a binder, which is typically a polymer such as polyvinyl butyral, and handling aids such as wetting agent and xe2x80x9cplasticizerxe2x80x9d. The solvent should be one that dissolves all of the organic components, is not reactive with any of the components, either organic or inorganic, and has a high vapor pressure so that it dries quickly. In practice, the solvent is frequently a mixture of solvents of slightly differing properties.
The slurry is deposited on a support strip by a xe2x80x9cdoctor bladexe2x80x9d, by curtain coating, or other applicator means. The support strip, with applied coating, is moved along a path that is long enough for the tape to dry by evaporation of the solvent before the tape is stripped from the support strip or is taken up on a storage reel still on the support strip. This dry product, called green tape, can be printed with conductive paste, punched or cut into desired shapes, and stacked and laminated into thick forms. The resulting structure is fired. During firing, the remaining binder and other organics are burned away and the inorganic components undergo melting, sintering, or chemical reaction to form a monolithic ceramic or glassy object.
Because the fired ceramic generally is required to be dense and pore-free for the sake of its electrical properties, a minimal amount of binder in the slurry is desirable. The more binder that must be burned to gas and released from the ceramic during firing, the more porosity and defects the fired article will have. Unfortunately, slurry with a low binder content is more likely to crack as it is dried to green tape.
In 1961, the greatest difficulty with Park""s method was drying the tape quickly enough to manufacture the tape in a continuous strip without needing to provide a prohibitively long path for the tape to travel as the solvent dried. Park taught that drying could be accelerated by heating the slurry to a temperature less than the boiling point of the solvent used. xe2x80x9cThe factors of temperature and rate of drying are controlled by passing heated air in a countercurrent direction to the movement of the coated supporting tape through a drying chamberxe2x80x9d. In 2001, Cappabianca teaches, xe2x80x9cA major limiting factor of tape casting is the time required to dry the tape. The surface texture of the tape directly relates to the drying rates. This can cause the drying chamber to be as long as 35 meters depending on the drying ratexe2x80x9d.
Cappabianca further states, xe2x80x9cThe evaporation rate is typically regulated by controlling the heating or air flow over the tape. The surface texture of a tape cast material directly relates to the drying rate. If the top surface dries too quickly, a skin may form over the tape resulting in defects. Some of the defects that arise by improper drying include bubbles in the tape, tape distortion, and tape cracking . . . The continuous caster""s air flow normally starts at the exit of the drying chamber and flows in the opposite direction of the moving tapexe2x80x9d.
Mistler and Twiname, in Tape Casting: Theory and Practice, say on page 104, xe2x80x9cCasting machines are very simple in design, since they are really only elongated (in some cases) forced-air drying ovensxe2x80x9d. On page 113, xe2x80x9cAnother factor in airflow requirements during the casting of any flammable solvent(s) in the presence of heat is safety. There are published regulations as to the volume of airflow that must be maintained to remain below the lower explosion limit (LEL)xe2x80x9d. Page 115, xe2x80x9cAll fans used on or near the tape casting machine should be explosion-proof if volatile, flammable solvents are being used in the processxe2x80x9d.
These references show that blowing heated air over the drying slurry in a countercurrent direction has two serious problems. Firstly, there is a grave safety and environmental hazard produced by mixing volatile (fast-drying) solvents with air. Tape casting operations are notorious for explosion and fire hazard, and workers are typically exposed to vapors of toxic chemicals. The means used to heat the air can add to the explosion danger. Solvent fumes that were once simply vented to the roof are now regulated and must be removed from the air by distillation, filtration, or ignition.
Air is typically blown over the slurry at a speed of 1 to 10 feet per second in order to maintain the solvent vapor at a concentration below the lower explosion limit. As a rule of thumb, the ratio of air mass to solvent vapor mass should be about one hundred. Keeping the concentration (partial pressure) of solvent vapor this low also increases the evaporation rate of the solvent from the surface of the slurry. This leads to rapid formation of the xe2x80x9cskinxe2x80x9d of binder on the upper surface of the slurry, as mentioned above by Cappabianca. The skin inhibits further evaporation and may cause cracks, wrinkles, wedging, and other defects in the tape.
Secondly, the electronics industry increasingly requires tape that is thicker, more uniform in thickness, and has fewer and smaller defects. At the same time, new materials are being introduced that are more difficult to cast thickly, such as ferrite for inductors. The usual method of using ferrite by the green tape process is to cast it into tapes less than 0.005xe2x80x3 in thickness, which are stacked together to form a layer of the required thickness. Attempting to cast the tape to the desired thickness with industry standard equipment can result in disastrous cracking and other defects.
To help decrease the defects induced by high airflow, many casting machines need to have baffles, louvers, and dampers to create local pockets of air with relatively higher concentration of solvent vapor when casting tapes thicker than about 0.010xe2x80x3 thickness. These devices are adjusted by trial and error and their interactions are not well understood.
Onur et al. (U.S. Pat. No. 5,212,877) teach that use of heated nitrogen instead of air to dry a coating such as magnetic tape avoids explosion and fire hazards, thus allowing the partial pressure of solvent vapor to be greater than the explosion limit in air, thereby moderating the evaporation of solvent and suppressing skin formation. Onur et al. teach discharging the nitrogen from nozzles above and below the support strip; the jets of gas float the strip in the stream to give quick, uniform drying. This method is not suitable for coatings as thick as green ceramic tape. It has been found that these thicker coatings tend to crack when heated from above. Also, rapid jets of gas impinging on the wet slurry would cause irregularity of the thickness because of the rheology of ceramic slurry.
Attempts have been made to use halogenated solvents such as methylene chloride or 1,1,1-trichloroethane as the solvents in ceramic slurries to avoid flammability problems, but halogenated solvents are expensive, toxic, highly regulated, and are incompatible with the binders and other vehicle components generally used.
Attempts have been made to develop vehicles using water as the main solvent. Water is wonderful for avoiding explosion, health, and environmental problems, but has limited use in ceramic slurries. Water has a low evaporation rate, the binders it is compatible with tend to be difficult to laminate and fire, and it reacts chemically with many of the oxides and other components used to make the ceramic.
Hebels (U.S. Pat. No. 5,1 68,639) discloses a method and apparatus for applying a coating to a web, such as glue onto adhesive tape, that facilitates recovery of the evaporated solvent vapor. Hebels desires the partial pressure of solvent vapor to be high to increase efficiency of the solvent recovery, and avoids fire hazard by drying the coating in a chamber previously flushed with nitrogen gas. Hebels has noted that a problem associated with high partial pressure of solvent vapor is condensation of the solvent on the inside of the chamber, therefore the apparatus disclosed includes a chilled roller inside the chamber, which preferentially attracts condensation. A stripping device diverts the condensed solvent into a trough so that the liquid does not drip onto the wet glue. A flow sensor monitors escape of solvent from the system and calls for increased chilling of the roller when solvent vapor escape is excessive. This apparatus and method does decrease the amount of volatile solvent released to the atmosphere, but does not have sufficiently sensitive control means for casting thick, high quality ceramic tape. Movement of the solvent vapor is weakly driven by entropy, and the only control means, the temperature of the roller, is too crude, slow, and indirect a means for achieving sufficient throughput of ceramic tape.
Mistier and Twiname note on page 154, xe2x80x9csome practitioners opt to slow the surface drying rate of their cast by placing solvent-soaked rags or paper towels or containers of solvent in the drying chamber to saturate the local atmosphere. While effective, this practice is not advisable due to fire and safety concerns; it would send your safety engineer and local OSHA representative into fitsxe2x80x9d. However tantalizing this tip is, the authors teach no way of accomplishing the same effect safely. In fact, they go on to say, xe2x80x9cSaturation or proximity to saturation is generally not an addressable concern with tape casting equipment due to the high levels of airflow. Airflow must be adjusted, apart from drying concerns, to a level specified by fire and safety codesxe2x80x9d. The section closes with, xe2x80x9cDue to the minimum airflow requirements imposed by safety and fire regulations, the surface evaporation can only be increased to a certain extent. Due to this limitation, the other variables affecting surface evaporation require attentionxe2x80x9d. Subsequently, Mistler and Twiname discuss customizing the formulation of the organic vehicle and the inorganic components of the slurry to ameliorate problems such as cracking and curling, especially increasing the amount of binder. As noted above, though, increasing the binder can lead to other problems.
The tradeoffs between drying rate and coating quality are problems in other applications, such as coating photographic film with emulsion or metallizing plastic film by depositing a metallo-organic solution that is subsequently converted to metal.
There is a long felt need for an apparatus and method for depositing and drying coatings containing volatile solvents that produce substantially defect-free coatings with uniform thickness, while minimizing health and safety risks. There is further a need for an apparatus and method that can produce large quantities of uniform product, including ferrite of a thickness greater than 0.005 inch.
There is a need for an apparatus and method that has sensitive control means for optimizing the process, yet can be operated by a typical technician. There is a need for such apparatus and method to include means for automated feedback and improvement of the process.
It would be desirable for such an apparatus to be adjustable to work well with a variety of formulations and thicknesses of slurry. It is desirable that the apparatus be adjustable to accommodate reasonable batch-to-batch variation and variations deliberately introduced for process improvement studies. Because casting machines are expensive and usually require elaborate equipment verification when installed, it is desirable to improve the performance of existing equipment by retrofitting only a new drying tunnel.
In the forty years separating Park, Jr. and Cappabianca, enormous changes have been seen in the materials and specifications for green ceramic tape, yet the apparatus and method for producing the tape have changed very little.
The present invention is an apparatus and method for drying ceramic green tape made from a variety of materials, in a wider range of thicknesses than can be achieved with prior apparatus and methods. The apparatus and method, with slight modification, is also useful for metallizing plastic film by coating with a metallo-organic solution, as well as other applications.
The apparatus of the present invention is a drying tunnel for a tape casting machine that includes a casting head and a movement system for driving a continuous support strip relative to the casting head. The drying tunnel comprises two chambers, separated from each other by a barrier. Within the first chamber, the slurry loses 80 to 90 per cent of the solvent by evaporation and changes from a fluid to a gelled state. In the second chamber, nearly all of the remaining solvent is evaporated and the coating that emerges from the tunnel is dry to the touch and has no odor of solvent.
Air is excluded from both chambers and an inert carrier gas, preferably dry nitrogen at ambient temperature and with a velocity typically 1 to 5 inches per second, helps control the rate of evaporation of the solvent from the slurry. The mass flow rate of nitrogen used is typically equal to the mass flow rate of solvent evaporated in the tunnel, so cost of nitrogen is not prohibitive. The carrier gas is introduced by a gas inlet in each chamber. A first gas inlet is located near the entrance to the tunnel and a second gas inlet is located near the exit from the tunnel.
A coating containing a volatile liquid, such as ceramic slurry containing an organic solvent, is applied to the support strip by an applicator such as the casting head of a casting machine. The strip and its coating of slurry then pass into the drying tunnel.
The floor of the tunnel is heated. Heat is conducted through the strip and raises the temperature of the slurry, with the highest temperature being always at the bottom of the slurry. Solvent diffuses upward through the slurry and evaporates from the upper surface. The vapor is entrained in the carrier gas and moved toward a gas outlet. Plug flow of carrier gas is maintained in the tunnel so that smooth and stable gradients of temperature and partial pressure of solvent vapor can be achieved.
In a preferred embodiment, the two chambers are separated by a barrier having a gap only sufficient to allow passage of the support strip and applied slurry so that exchange of gases between chambers is minimized. Each chamber has a gas outlet and associated solvent vapor removal means (described below).
The atmosphere inside the first chamber has a high partial pressure of solvent vapor, and is nearly saturated. To prevent solvent from condensing on the upper part of the tunnel and damaging the slurry coating by dripping onto it, or obscuring the view through the cover of the tunnel, the tunnel cover is heated slightly.
The high partial pressure of solvent vapor in the first chamber moderates the evaporation of solvent from the surface of the applied slurry, so that the binder does not form a skin before the slurry has gelled.
To allow drying to progress, solvent vapor must be removed from the tunnel. The carrier gas guides the vapor toward the gas outlet to a solvent vapor removal means, such as a condenser.
A first gas outlet is located in the first chamber near the barrier. A second gas outlet is located in the second chamber, near the other side of the barrier. Carrier gas is thus moving from both ends of the tunnel toward the barrier. As a result, carrier gas flow is parallel to the direction of travel of the support strip in the first chamber and counter-directional to the travel in the second chamber.
The apparatus of the present invention has a great many variables that can be adjusted to produce the best results. This sensitivity of control allows many types of ceramic and glassy materials to be cast to dry thicknesses greater than 0.005xe2x80x3 thick with uniformity of thickness on a single machine with a relatively short tunnel length. These variables interact predictably; with suitable sensors in place and an appropriate algorithm, sophisticated automated control is practical. The design of the system allows existing casting machines to be retrofitted to use the method of the present invention.
Because a solvent removal system, such as a condenser, is an integral part of the apparatus, solvent is not released to the ambient atmosphere and a burn-off torch is not needed. The solvent recovered from the condenser is typically reused. The carrier gas may be recirculated to the drying tunnel after solvent vapor is removed.
The apparatus of the present invention uses a fraction of the electricity that the drying tunnels of casting machines now in use in the industry do because enormous volumes of air are not heated. No explosion-proof air fans and heaters are required.